RACHEL MARTIN, HOST:
Salmon are dying in rivers across western Alaska. The largest die-off reportedly comes from the Koyukuk River, a tributary of the Yukon. And as KYUK's Anna Rose MacArthur reports, scientists suspect the summer's record heat is the cause.
ANNA ROSE MACARTHUR, BYLINE: In mid-July, Lisa Bifelt needed to catch fish to eat over the winter. So she and her boyfriend boated to an eddy near her home in Huslia on the Koyukuk River.
LISA BIFELT: And then that's when we started noticing the dead salmon. Like, every place we would stop, they would be floating.
MACARTHUR: They counted hundreds of dead summer chum. Stephanie Quinn-Davidson directs the Yukon River Inter-Tribal Fish Commission. She flew a team of scientists to the Koyukuk to check out what was happening and recorded videos.
STEPHANIE QUINN-DAVIDSON: Dead salmon washed up, just piles of them.
MACARTHUR: The salmon had not spawned. They had underdeveloped eggs and sperm, meaning they still had a long way to go upstream.
PETER WESTLEY: A river that is usually teeming with life felt like a tomb.
MACARTHUR: University of Alaska Fairbanks fisheries professor Peter Westley was also on the trip. The salmon showed no signs of parasites, lesions, tumors or other infections. And because other fish species were not dead, pollution and low oxygen seemed unlikely.
WESTLEY: Other than being dead, looked fairly healthy.
MACARTHUR: Vanessa von Biela studies the effects of temperature on Yukon salmon.
VANESSA VON BIELA: I think there's strong evidence to suggest that fish are dying because of heat stress.
MACARTHUR: Her research shows temperatures approaching 70 degrees begin demanding enormous energy from the fish.
VON BIELA: So it's like if you start a long road trip, and you actually end up running out of gas before you make it to your destination.
MACARTHUR: The river spent more than a month at or above this threshold. There were temperatures so warm they had never been recorded on the Yukon. Holly Carroll was also on that trip. She's with the Alaska Department of Fish and Game, and it's her job to manage where and when people can fish for summer chum and how much they can catch. She also suspects the warm water was responsible for the die-off.
HOLLY CARROLL: It just was probably too much for them.
MACARTHUR: And she thinks it's disconcerting, but she says...
CARROLL: There's 1.4 million salmon that came in this river, and we - you know, we might have seen thousands die off, but I'm not worried about the future of this species.
MACARTHUR: She says salmon are resilient, but it's their resilience that alarms another scientist on the trip, UAF professor Peter Westley.
WESTLEY: They're really tough. And so if salmon are dying, it points to something fairly serious.
MACARTHUR: Despite the die-off, subsistence families like Lisa Bifelt's, who rely on a certain amount of summer chum to put away for winter, have not reported difficulty meeting their harvest schools, and king salmon, which are larger, have not been reported dead.
For NPR News, I'm Anna Rose MacArthur in Bethel. Transcript provided by NPR, Copyright NPR.